Magnetic analysis method



April 13, 1937. T zuscHLAg; 2,077,161

MAGNET I C ANALY S I S METHOD Filed March 23, 1934 INVENTOR TheodorZudch/ag ATTORNEYS Patented Apr. 13, 1937 UNITED STATES PATENT OFFICEMAGNETIC ANALYSIS METHOD Theodor Zuschlag, Englewood, N. J., assignor toMagnetic Analysis Corporation, Long Island City, N. Y., a corporation ofNew York Application March 23, 1934, Serial No. 716,942

8 Claims.

.7 pensation of forces which tend to interfere in the determination -ofmetallurgical or physical properties of magnetizable bodies.

' According to one practice in the heretofore customary art of magneticanalysis, a specimen 1: to be tested and a standard magnetizable bodywhose metallurgical or physical products are' known, are simultaneouslysubjected to the infiuence of primary fluctuating electromagnetic fieldsproduced by a common source of alterl3 nating current. A pair ofsubstantially identical secondary coils are placed in inductiverelationship with the standard and the specimen respectively, and areconnected in series opposition with an indicating means so that the in-30 duced differential current in the two'secondary coils may be measuredand taken as a criterion to establish the differences in metallurgicalor physical properties between the specimen and the standard. This typeof practice may well be 5 described as a comparative system of magneticanalysis in which alternating current is used. In such practice,amplitude, frequency, phase and wave forms of the induced differentialcurrentare indicative of certain properties whose pres- :m ence orabsence is sought to be determined.

5:5 carrying out investigations for industrial purposes, this conditionis not always realized. Frequently the differences between specimens areof such magnitude that the limit of the indicating means is exceeded. Insuch a case two courses in are open. The sensitivity of the instrumentmay be reduced to keep the reading within the scale of the indicator, orcompensation of the difference may be resorted to. For obvious reasons,reduction in sensitivity is seldom desirable, and com- 45 pcnsation ofthe differences is the desirable al ternative. The chief purpose of thisinvention is the compensation of differences between specimen andstandard which tend to interfere in the determination of metallurgicalor physical 50 properties of the magnetizable bodies underinvestigation.

In the ideal case, the various parts of the apparatus are so constructedand disposed with respect to each other that electromagnetic forces 5created in them do not interfere with each other.

In the case of apparatus suitable for industrial purposes, however,stray currents, variations in supposedly identical parts of equipment,unsymmetrical disposition of wiring and other factors lead to thepresence of extraneous forces that tend to'mask the true differencessought, and vitiate in large measure the accuracy of results.

For this reason it is necessary in the case of practical workingapparatus to employ means whereby both variations between specimens andthe effect of extraneous forces may be compensated for or eliminated inorder that the results obtained shall be accurate and shall offer a truebasis for the determination of the physical or metallurgical propertieswhose presence is to be discovered.

As a result of my investigations I have discovered thatby means of anovel bridge network in a magnetic analysis circuit such as has beendescribed I am able to compensate for, or neutralize, the effect of bothvariations between specimen and standard and the extraneous forces whosepresence in the secondary circuit tends to obscure or vitiate the truediiferences between the properties of the specimen and the standardunder investigation.

In another aspect, my invention contemplates the suppression ofharmonics whose presence tends to confuse rather than to simplifyresults, so that the resulting induced differential current of thesecondary circuit may be measured by simple indicating means such asgalvanometers. In this way a true indication of the differences inmetallurgical or physical properties between standard and specimen maybe obtained with reat accuracy and by a simple mode of operation. Theapparatus of my invention, in addition to permitting greater accuracy ofmeasurements, is rugged in construction and suitable for the severeconditions which are met in the industrial applications of magneticanalysis. The mode of operation is sufiiciently simple that extremelyaccurate results may be obtained by relatively inexperienced operators.A further advantage is presented in that the indicating Fig. 1 is apresently preferred form of magnetic analysis apparatus in which isincorporated a novel compensating net-work of my invention.

Fig. 2 is a further refinement of the magnetic analysis apparatus of myinvention in which is incorporated both the novel compensating networkdescribed herein and the wave analysis mechanism shown in my co-pendingapplication, Serial No. 716,941 filed March,23, 1934.

Referring to Fig. 1, it will be observed that two substantiallyidentical primary coils, I and 2, of relatively low ohmic resistance areconnected in series with a voltage supply transformer 3, an ammeter 4,and a low ohmic noninductive fixed resistance 5. The primary coils arepreferably constructed of a suitable number of turns of insulated copperwire of low ohmic resistance. The voltage supply transformer ispreferably of the adjustable auto-transformer type. The input leads ofthe transformer are connected to a suitable alternating current source6.

A pair of substantially identical secondary coils l3 and I4, preferablycomposed of a suitable number of turns of insulated copper wire of lowohmic resistance, are connected in series opposition to the contact armsof a double-pole three-position switch IS. The pair of secondary coilsare disposed in inductive relationship with the primary coils,preferably by placing each secondary coil within a primary coil in axialalignment therewith. By connecting the two poles of switch l5 to contactpoints I! and I8 the two secondary coils are connectible directly to theend points of a potentiometer l6. Placing the double-pole switch on thecontact points 2| and 22 serves to connect the secondary coils in serieswith the fixed resistor lB, the end points of potentiometer I6 and thevariable resistor 20. By means of the single-pole triple-throw switch25, the slider 23 of the potentiometer IB is connectible through contactpoint 24 to a point on the direct wire between the secondary coils, thispoint being hereinafter referred to as the common point 48. The slider23 of potentiometer 16 may also be connected to the common point 48through fixed resistance 26 and contact point 21 of switch One end ofthe fixed resistor 5 of the primary circuit is connected in series withthe milliammeter 1 to an end of a center tap potentiometer 8. The otherend of fixed resistor 5 is connectible to the other extremity ofpotentiometer 8 through a single-pole double-throw switch l2, eitherdirectly through contact point 9, or in series with afixed resistance Inthrough contact point II.

The slider 49 of center tap potentiometer 8 is connected to an end ofpotentiometer I6, .this end of potentiometer l6 being also connectiblethrough contact points I8 or 22 to the secondary coils as has beendescribed above. The other end of potentiometer IS, in addition to beingconnectible with the secondary coils through contact points I! or 2|, isconnectible in series with an inductance 28 and a capacitance 29 to theslider 3| of a potentiometer 32. The inductance 28 and capacitance 29serve the function of a filter and are preferably constructed to passonly the first harmonic of the primary circuit, but may, if desired, beconstructed to pass the third or any other harmonic.

The center tap of potentiometer 8 is connected in parallel with asuitable current indicating means 41 and an end of potentiometer 32, the

other end of potentiometer 32 being connected in series to the secondlead of the indicating means.

The functions of the various parts of the apparatus may be understoodmore easily by considering the various modes of operation in connectionwith the drawing. In all cases the secondary coils I3 and I4 are firsttested for balance with no magnetizable bodies inserted in the fields ofthe primary and secondary coils. To accomplish this, switches I2 and 25are placed in the center or ofi' position and switch I5 is closed overcontact points I! and I8. With the apparatus connected in this mannerthe potentiometer It is connected across the secondary coil circuit butslider 23 is not connected to the common point 48 of the circuit, sothat any voltage drop across the potentiometer must be due tounequalelectromctive forces induced in the secondary coils. A voltage dropacross potentiometer 5 causes current to flow in the circuit I6, 28, 29,3|, 32, 8 and 49, which is a closed circuit depending for energizationupon the current flows in the several potentiometers. The existence ofany current flow in this circuit is immediately indicated by adeflection of the indicator 41, which shows the voltage imposed betweenslider 3| and the common end of the potentiometer 32. It should beobserved that in the case of properly designed secondary coils which arelocated coaxially with respect to the primary coils, the resultantdeflection of the indicator should be small.

-In order to compensate for any deflection of the indicator, switch 25is closed on contact point 24, thus connecting coils l3 and I4 in bridgerelationship with the potentiometer. This bridge arrangement permits avariation of the ratio of the secondary currents induced in coils l3 and14.

Thus the strength of the secondary potentials in these coils may bevaried by adjusting slider 3| of potentiometer 32, while an adjustmentof slider 23 of potentiometer l6 permits a reduction in the strengths ofthe potentials induced in the secondary coils to minimum values. In manyinstances, however, the minimum value attainable by the adjustment ofpotentiometer I6 is not zero,- and in order to obtain a zero reading ofthe indicator it is necessary to close switch l2 to either of the twocontact points It! and 9 and adjust the slider 49 of potentiometer 8. Itwill be observed that the auxiliary drop thus imposed acrosspotentiometer 8 between the center tap and the slider 49, issubstantially out of phase with the open circuit air core potentials ofthe. secondary cores. This is due to the source of this potential dropin the primary circuit. Secondary phase components in the secondary coilcircuit which are so disposed as to be'unafiected by adjustment ofpotentiometer I6, may be neutralized by means of the 90 phase displaceddrop of the potentiometer 8.

The range of compensation obtainable by potentiometers 8 and 16 may bevaried by inserting resistances I0 and 26 into their respective circuitsby means of switches I2 and 25, and this increased range of compensationis obtained without altering the sensitivity of the apparatus to anappreciable degree. It should be understood that while the resistors i0and 26 are not absolutely essential to the operation of the compensatingnet-work, they are desirable adjuncts because of the additional range ofcompensation which they provide. Usually in the case of air core testingsuch as has been described, a well designed apparatus will requirelittle compensation and zero readings will be obtained with the slidersclose to the midpoints of potentiometers l6 and 8.

When the indicator reads zero with no magnetizable bodies in the fieldof primary and secprimary and secondary coils, say I and I3, and a aspecimen B, whose properties are to bedetermined, is inserted in thefield of the other pair of primary and secondary coils, say 2 and H. Theindicator will be deflected in proportion to the differences in themagnetic properties oi the standard and the specimen. The reading of thespecimen B in the coils, and the deflection correindicator is noted. Ifseveral specimens are to be tested these are substituted successivelyfor sponding to each specimen is noted. These readings are indicative ofthe difierences in metallurgical or physical properties between specimenand standard, or between specimens themselves.

When specimens and standard difier so markedly in magnetizablecharacteristics as tocause a deflection which is too great to be readonthe scale of the indicator, as is frequently the case when specimensand standard represent different heats of metal, or when eitherspecimens or standard contain flaws of considerable magnitude, thetesting procedure is somewhat difierent. The change in procedure is madenecessary by the fact that the setting of potentiometer 16 for a zeroreading under air core conditions must be changed in order toaccommodate the reading upon the scale of the indicator. The procedurein such a case is as follows:

A standard A and a specimen B are inserted in the field of the coils aspreviously described, and the resultant deflection of the indicator isreduced to zero by adjustment of the potentiometers 8 and I6. Thesettings of the potentiometers 8 and I6 are noted. Specimen B isreplaced by a second specimen C and the potentiometers 8 and I6 areagain adjusted until a zero reading is obtained. The settings of thepotentiometers for specimen C are noted. A comparison of the settingsfor the two cases serves as a basis for diflerentiating specimens B andC, insofar as their magnetic characteristics are concerned. It will beobserved that this method of procedure, while producing results whichare as trustworthy as those obtained in the previous methodgisconsiderably slower because it requires considerable manipulation. Forthis reason it is preferable to choose a standard whose magnetizablecharacteristics approximate those of the specimens to be tested,especially in the case of quantity production.

The procedures thus far described find their chief utility in thequalitative testing of steel specimens where one or more defects of aparticular kind are sought to be discovered by comparison with astandard in which these defects (10' not occur. The utilityof-theapparatus is not limited, however, to qualitative determinations.The apparatus may also be used to establish a quantitative comparison ofmagnetizable bodies in that the total magnetic loss difference and theratio of the apparent permeabilities of two samples may bequantitatively determined with a high degree or accuracy. The procedure,

in the event that as follows:

The two samples to be compared are inserted respectively into the pairsof secondary and primary coils. The circuit is then balanced byadjusting potentiometers 8 and I 6 to obtain comthese quantities aresought, is

. plete neutralization, when the indicator willreg- 'ister zero. Whenthis condition is obtained it ences is simply a matter of substitutingthe values in the following equation and solving it for W; i. e. 1

W=ir

where; W is the total magnetic loss diilerence between the two samples,expressed in watts.

i is the efiective current flowing in potentiometer 8 in amperes and, ris the resistance between the center point of the potentiometer 8 andthe slider 49, in ohms.

The determination'of the permeability ratio is somewhat more complicatedthan the determination of l the total loss difference between thesamples. It is based upon the determination of ratio of the secondarypotentials induced in coils l3 and H. In order to determine thispotential ratio, two successive compensations, for slightly differentcircuit conditions, are necessary. These two compensations areaccomplished in the fol-- lowing manner:

The samples to be compared are inserted in the fields of the secondarycoils. The contact arms of the switch l5 are connected to contact points"and I8 and the contact arm of switch 25 is connected to the contactpoint 24. With the apparatus in this condition the potentiometers 8 andI8 are adjusted so that the indicator Substituting the values of I14 andI13 from'Equations 1 and 2 in Equation 3':

E14 17 Uri-( 16 It will be noted that Equation 4 contains the unknownimpedances m and m which must be eliminated before the numerical valueof the ratio Eu/Eis can be calculated. .In order to acresultant currentin secondary complish this elimination it is necessary to obtain asecond series of values for another compensated condition. This secondcompensation procedure consists in reversing switch I5 and placing it incontact with points 2| and 22 so that the fixed resistor l9 and thevariable resistance 20 are inserted in the circuit. The circuit is thencompensated by adjusting; the resistance 20 without changing theadjustment of the potentiometer l6. New current values will naturally beobtained in the secondary coils. These new current values are designatedin the following equations as I14 and I13. The other values aredesignated as in the previous Equations 1, 2, 3 and 4:

These equations may be combined as follows:

E r b 'l4+ l9 it b m It will be observedthat Equation 9 contains nounknown factors since the values I), m, m and T are known. The numericalvalue of the equation may therefore be easily computed. Knowing thisvalue it is possible to compute, by means well known in the art, theratio of the apparent permeability of the tested specimens.

In conducting tests with the apparatus of my invention it is preferablethat an eflicient filter combination 23, 29 be used. It is not essentialthat a filter of the particular type shown be employed, but whatever thetype of filter, the aim is to eliminate all but one harmonic of the basefrequency in order to suppress harmonics whose presence do not aidin thedetermination of magnetic characteristics. It has been found that mostdefects and properties which are sought to be determined by magneticanalysis appear in the first and third harmonics of the basefrequency.Accordingly filters adapted to pass only one of these two frequenciesare preferable. It should be noted, however, that the apparatus is notlimited to an investigation of the basic and third harmonics of theexciter current. Occasionally it may be desirable to investigate otherharmonies, and this may be easily accomplished by changing theinductance and the capacitance of A the filter 2a, 29.

An-outstanding advantage accures-to the use of the compensating means ofthis invention when it is employed in combination with the twocomponentindicator of the invention which is described in my co-pending U. S.application, Serial No. 716,941, filed March 23, 1934.

.Fig. 2 shows a compensating magnetic analysis circuit incorporatingboth the compensating means described and the two-componentindi-- catorof my aforementioned application. It will be observed that thistwo-component indicator been substituted-for the indicating means 41{-Jshownon Fig. 1 A synchronous motor 38 de- "rives power from thecurrent source 6 ahead of the autotransformer 3, and is operably as-'sociated with commutator 31. The ends of the potentiometer 32,previously reterred to in the description of Fig. 1, are connected tobrushes 34 and 33, the brushes being in contact with contact rings 35and 36, respectively, of the synchronous commutator 31. When thesynchronous motor is of the two-pole type the commutator 31 is equippedwith two separate contact segments 39 and 40 which are connected to thecontact rings 35 and 36, respectively. Two pairs of diagrammaticallyopposed brushes 4|, 42, and 43, 44 are symmetrically disposed upon thecontacts 39 and 40 and are connected to two D'Arsonval typegalvanometers 45 and 46. The two brush pairs are 90 phase displaced andmay be moved in unison along contact segments 39 and 40.

When this two-component type of indicator is used the potential dropbetween the two ends "ofpotentlal 32 may be divided into two componentswhich are 90 phase displaced with respect to each other., The voltagedrop between slider 3| and the one end of the potentiometer 32 istransferred to contact rings 36, 35 and return to the contact segments39, 40. The brush pairs 4|, 4! and 43, 44 receive the rectified voltagefrom the contact segments and deliver it to the indicators 45 and 46. Inother words, the voltage wave of the one complete cycle is broken upinto two currents 90 phase displaced, integrated and indicated withregard to their arbitrary origins. The simultaneous indication of thetwo phase displaced components permits the reconstruction of the voltagewave, a feature which normally may be accomplished only with the help ofan oscillograph.

It will be apparent that when carrying out tests with the apparatus,including the two-component indicator, it is essential to use aneflicientfilter combination-28, 29, in order to pass only the harmonicwhich is being investigated. In order to further describe the processinvolved in my new apparatus and method for magnetic testing in which atwo-component indicator is used in conjunction with the compensatingmeans which has been described, the following is set forth:

in .series with the segments of commutator 31.

The commutator serves to break up the cycle differential current intotwo 90 phase displaced currents which are separately integrated andshown by the indicators. obtained only when both the indicators readzero. By means of the two-component indicator it is possible toreconstruct the wave form of the differential current in a manner whichpreviously was impossible without the use of oscillographs.

When the two-component indicator is used in conjunction with thecompensating net-work of my invention it is possible. to determine bymeans of one indicator the presence of a total loss difference and itsmagnitude, while the. companion indicator will reflect the ratio of thepotential differences induced in the two secondary coils. Investigationhas shown that the readings of the two indicators may be well calibratedby comparing them with the potentiometer readings, so that actuatevalues of. both potential ratio and total True neutralization istosuppress all but one harmonicJ' loss difference may be determineddirectly on the indicators.

I claim:

l. The method of magnetic analysis which comprises setting upalternating fluxes in a primary circuit by means of alternating current,inducing two secondary currents in a secondary circuit placed ininductive relationship with said primary circuit, opposing said currentsin the secondary circuit, neutralizing the resultant differentialcurrent in said secondary circuit by varying the resistance thereof andby introducing thereinto an auxiliaryprimary current in phase with theprimary current, inserting a standard of known magnetism affectingproperties and a specimen to be tested in inductive relationship withthe secondary circuit, breaking up the resultant secondary current intotwo components and integrating the two components. I

2. Method according to the preceding claim in which the secondarycurrent resulting from the insertion of standard and specimen ininductive relationship with the secondary circuit is filtered,

3. The method of magnetic analysis which comprises inducing analternating flux in a standard of known magnetism afiecting propertiesand in a specimen to be tested by means of a common alternating currentsource, utilizing'said fluxes cuit placed in inductive relationship withsaid specimen and said standard, opposing the secondary currents in saidsecondary circuit, neutralizing the resulting differential current byvarying resistances in said secondary circuit and by introducingthereinto an auxiliary primary current in phase witlnthe primary currentsource, noting the magnitude of the resistances in the secondary circuitwhen .the neutral current condition prevails, replacing'the specimen bya second specimen and again neutralizing the differential cur-' rent inthe secondary circuit by changing the magnitude of the resistancestherein and byadjusting the amount o1 auxiliary current introduced intothe secondarycircuit and noting the magnitude of the resistances in thesecondary circuit when the second neutral condition prevails. 4. Themethod of m flntic analysis which comprises simultaneously setting up an-alterhating magnetic flux infa standard of. known magnetizablecharacteristics andin a specimen to be tested by means ofaprimaryalternating current, inducing two secondary currents in asecondary circuit placed in inductive relationship with said specimenand said standard, opposing the secondary currents in a secondarycircuit, neutralizing the secondary currents in the secondary circuitdue to forces-other than those which reflect the difierences' inphysical properties of standardfand specimen which are sought to beinvestigated by appropriate adjustment of resistances in the secondarycurrent and by intro- 5. The method .of magnetic analysis whichcomprises establishing twp alternating fluxes by means of a primaryalternating current, inducing two secondary currents in a secondarycircuit disposed in inductive relationship with the alternating fluxes,opposing said secondary currents in the secondary circuit, neutralizingthe resultant diflerential current in thesecondary circuit ;.wli'en nomagnetizable bodies are inserted within "the fields of the alternatingfluxes by varying the magnitude of resistances in thesecondary circuit%nd by introducing primary current into the sec on'dary circuit,introducing a standard of known circuit after adjustment, replacing thefirst speci-' men by a second specimen to be tested and againneutralizing the resulting diiierential current by adjusting themagnitude of the resistances in the secondary circuit, again noting themagnitude of the resistances in the secondary circuit and comparing themagnitude of the resistances corresponding to the first and secondspecimen.

' 61. In a method of magnetic analysis in which a primary-alternatingcurrent is utilized to into induce secondary currents in a secondarycirintegrating the components and producing the integrated componentsfor inspection.

" 7. The method or magnetic analysis which comprises setting up analternating flux in a pri; mary circuit by means of a primaryalternating current, inducing a secondary current in a sec-.ondarycircuit placed in inductive relationship with said primarycircuit, neutralizing the secondary current by introducing into thesecondary circuit an auxiliary primary current in phase i with theprimary current, then inserting a speci-.

men to be tested into inductive relationship with the secondary circuit,breaking up the secondary current resulting from the insertion oi thespecimen into, inductive relationship'with the secondary circuit intophase displaced components, simultaneously integrating the componentsand producing the'integrated components for inspection.

8. In a method 01' magnetic analysis in which a primary alternatingcurrent is utilized to induce a differential secondary current in asecondary circuit, the improvement which comprises introducing anauxiliary alternating current in phase THEODOR ZUSCHLAG.

